The present invention relates to a method for controlling the combustion process in a combustion engine. The invention especially relates to such a method for reducing soot emissions and nitrogen oxide emissions (NOx) formed in combustion engines in which the fuel/cylinder gas mixture is ignited by compression heat generated in the cylinder.
Nitrogen oxides (Nox) are formed from the nitrogen content in the air in a thermal process which has a strong temperature dependency and depends on the size of the heated-up volume and the duration of the process.
Soot particles are a product which, during combustion, can both be formed and subsequently oxidized into carbon dioxide (CO2). The quantity of soot particles measured in the exhaust gases is the net difference between formed soot and oxidized soot. The process is very complicated. Combustion with fuel-heavy, i.e. rich, fuel/air mixture with poor mixing at high temperature produces high soot formation. If the formed soot particles can be brought together with oxidizing substances such as oxygen atoms (O), oxygen molecules (O2), hydroxide (OH) at sufficiently high temperature for a good oxidation rate, then a greater part of the soot particles can be oxidized. In a diesel engine, the oxidation process is considered to be in the same order of magnitude as the formation, which means that net soot production is the difference between formed quantity of soot and oxidized quantity of soot. The net emission of soot can therefore be influenced firstly by reducing the formation of soot and secondly by increasing the oxidation of soot. Carbon monoxide emissions (CO) and hydrocarbon emissions (HC) are normally very low from a diesel engine. Yet the percentages can rise if unburnt fuel ends up in relatively cool regions. Such regions are, in particular, zones with intense cooling located close to the cylinder wall. Another example is cavities between piston and cylinder lining.
A combustion process in which the fuel is injected directly into the cylinder and is ignited by increased temperature and pressure in the cylinder is generally referred to as the diesel process. When the fuel is ignited in the cylinder, combustion gases present in the cylinder undergo turbulent mixing with the burning fuel, so that a mixture-controlled diffusion flame is formed. The combustion of the fuel/gas mixture in the cylinder gives rise to heat generation, which causes the gas in the cylinder to expand and which hence causes the piston to move in the cylinder. Depending on a number of parameters, such as the injection pressure of the fuel, the quantity of exhaust gases recirculated to the cylinder, the time of injection of the fuel and the turbulence prevailing in the cylinder, different efficiency and engine emission values are obtained.
Conventional combustion engines which work according to the diesel process exhibit relatively high values in terms of discharged emissions, such as nitrogen oxides and soot particles.
It is previously known to reduce the soot particle formation by injecting the fuel early in or prior to the expansion stroke, whilst, at the same time, an ignition delay is sought, so that the fuel has time to be vaporized and be mixed before the fuel is ignited with gases present in the cylinder. There are therefore methods for reducing the content of emissions which are given off from a conventional engine.
In order further to reduce the soot emissions specifically, a known method has been proposed, which entails the fuel being injected directly into the combustion chamber under relatively high injection pressure (up to 2000 bar has been tested) by means of injection devices disposed in the combustion chamber. The high injection pressure results in a high flow rate for the fuel relative to the injection device and the cylinder gas. The high flow rate of the fuel supplies energy to the mixing process between fuel and cylinder gas, which leads to a high mixing rate between these. When the mixing rate is sufficiently high, the chemical reactions between fuel and the cylinder gas which lead to combustion do not have time to occur, so that the combustion occurs farther into the combustion chamber. A large so-called xe2x80x9clift-offxe2x80x9d is obtained, i.e. a relatively large distance between the mouth of the injection device and the place downstream in the spray at which the fuel/cylinder gas mixture reacts. The large distance offers the opportunity for more cylinder gas and hence oxygen to be sucked in to the central parts of the spray. The result of the combustion occurring farther into the combustion chamber is that fuel and cylinder gas are mixed to a higher degree prior to combustion. When the mixing rate and the degree of mixing are sufficient, the combustion of the fuel/cylinder gas mixture is realized with a sufficient quantity of oxygen to reduce soot particle formation in the spray. At lower flow rates of the fuel, the oxygen in the cylinder gas is not mixed sufficiently well with the fuel prior to combustion, with the result that much of the combustion is realized with considerable oxygen deficiency. This creates large quantities of soot particles. According to this method, the mixing process between the fuel and the cylinder gas is primarily realized locally in the spray. The drawback with this method is principally that the nitrogen oxide emissions are not sufficiently low, but also the soot emissions can be somewhat too high. Without Exhaust Gas Recirculation, there is a lower limit for the minimum level of nitrogen oxide emissions which are obtainable. Lower nitrogen oxide emissions are most commonly achieved by postponing the time for the start of the injection during a combustion cycle. The later after the upper dead centre of the piston the combustion occurs, the lower the compression temperature. In such a case, combustion is realized at a lower temperature, thereby producing lower nitrogen oxide emissions. However, the postponed start of the injection influences the time available for completion of the combustion. Time available for soot oxidation is reduced for the same reasons. The result is that the soot emissions from the engine increase more and more the later that combustion occurs. This therefore limits the minimum level of nitrogen oxide emissions which can practically be reached without Exhaust Gas Recirculation.
The majority of measures which reduce soot emissions increase the nitrogen oxide emissions. A xe2x80x9ctrade-offxe2x80x9d is talked of, which is typical of the diesel engine, between soot emissions and nitrogen oxide emissions, which xe2x80x9ctrade-offxe2x80x9d is difficult to influence.
In order further to lower the emissions, a known method has been proposed which involves the fuel being injected at low pressure into the charge-air system of the engine during a certain time window, for example during the early part of the induction stroke. During the induction stroke, a large quantity of exhaust gases is also recirculated to the cylinder, this in order to cool down the combustion process to prevent nitrogen oxides from being formed. The formation of nitrogen oxides occurs at high combustion temperatures. The recirculated exhaust gases reduce the concentration of oxygen in the combustion chamber, due to the fact that a considerable proportion of the combustion chamber is taken up by re-introduced exhaust gases. A lesser quantity of oxygen leads to a cooler combustion process, but, at the same time, also to the formation of more soot emissions due to greater risk of local oxygen deficiency. The remaining quantity of oxygen must therefore be more effectively utilized in the reactions with the fuel. The solution to this is to supply more mixing energy, so that the oxygen xe2x80x9chits uponxe2x80x9d the fuel. This is done by making the recirculated exhaust gases and newly supplied air flow in a vortex motion, which is generally referred to as swirl, so that an essentially homogeneous mixture of fuel and cylinder gas is formed in the cylinder. When the piston then approaches the upper dead centre position, the homogeneous fuel/gas mixture is heated up by the compression heat generated in the cylinder. Since the fuel is fed into the charge-air system of the engine during the induction stroke, the fuel/gas mixture will be ignited once the entire quantity of fuel has been introduced into the cylinder and once ignition temperature has been reached in the cylinder. This combustion process is usually generally referred to as HCCI (Homogeneous Compression Combustion Ignition). The result is that very low contents of both soot and nitrogen oxide emissions are obtained. The drawback with this method is, however, that the engine can only be run at low loads. The reason is that the combustion of the entire quantity of fuel in a homogeneous fuel/cylinder gas mixture is realized far too rapidly, i.e. over relatively few crank angle degrees, thereby producing a far too rapid or sudden and strength-limiting pressure build-up and a maximum level of pressure in the cylinder. Another drawback is that the fuel/gas mixture located closest to the cylinder walls has time to be cooled and hence is not ignited by the compression heat, or alternatively that fuel which is burning close to the walls of the cylinder is extinguished due to cooling effects close to the walls. This results in some of the fuel condensing on the cylinder walls and passing by the piston. The fuel subsequently runs down into the crank section of the engine and is mixed with the lubricating oil of the engine, resulting in worsened lubricating properties of the oil. A further drawback with this method is that the ignition time for the fuel/cylinder gas mixture is difficult to control, especially with varying load and engine speed. The unburnt fuel/gas mixture also gives rise to the formation of hydrocarbons. Since the combustion process according to this known method is realized at relatively low temperature, the temperature of the exhaust gases will also be low. This has the effect of making the after-treatment of the exhaust gases more difficult, since the low exhaust gas temperature is not always capable of activating a catalyzer disposed in the exhaust gas system.
There is another known method which is a refinement of the method above and which involves the fuel instead being injected directly into the cylinder at a late stage during the compression stroke or early expansion stroke, according to which care is taken to ensure that the fuel/cylinder gas mixture is only ignited once the entire quantity of fuel has been injected into the cylinder. Here the recirculated exhaust gases and newly supplied air are made to flow in a very fast vortex motion, this firstly in order to obtain a homogeneous mixing with the fuel such that the oxygen is utilized effectively and secondly to prevent fuel condensation on the cylinder walls. In order for the mixing to be sufficient, the ignition delay, i.e. the time from the start of the fuel injection up to the ignition of the fuel/cylinder gas mixture, is extended. Here too, the result is that very low emission contents are obtained. Drawbacks with this method are principally that the engine can only be run at low loads, due to the fact that the combustion process of the entire quantity of fuel in a homogeneous fuel/cylinder gas mixture is realized far too rapidly, which produces engine strength problems. Here too, relatively cool exhaust gases are obtained, thereby making after-treatment of the exhaust gases with the aid of a catalyzer more difficult.
The object of the invention is therefore, in the light of the above, to control the combustion process in such a way that very low soot and nitrogen oxide emissions are obtained over the whole of the working range of the engine, i.e. at all loads and also during transient processes, such as, for example, during acceleration. The present invention also breaks the typical xe2x80x9ctrade-offxe2x80x9d (mentioned above) between soot and nitrogen oxide emissions and allows nitrogen oxide emissions to be lowered without increasing the soot emissions. The invention further ensures that high efficiency is achieved and that the cylinder pressure is kept at an acceptably low level.
The method according to the invention involves the spray, during the injection process, supplying a large quantity of kinetic energy and controlling a spray-internal mixing process and supplying kinetic energy to the large-scale global mixing process. At the same time, as a result of the motion and design of the piston, kinetic energy is supplied to the spray-internal and to the global mixing process.
The advantage with this is principally that low soot and nitrogen oxide emissions are obtained.
In a first advantageous embodiment of the method according to the invention, the soot and nitrogen oxide (Nox) emissions and the efficiency of the engine are controlled essentially independently of each other in that the soot emissions are primarily controlled by the quantity of supplied mixing energy and the nitrogen oxide emissions are primarily controlled by the quantity of exhaust gases from earlier combustion processes and the efficiency is primarily controlled by the centre of gravity and duration of the heat release. The advantage with this is that the emission contents and efficiency of the engine can, in principle, be freely chosen.
In an advantageous second embodiment of the method according to the invention, the mixing is carried out locally, since fuel and cylinder gas are mixed in regions upstream of the regions in the spray where combustion takes place and since the injection continues after ignition has been realized, i.e. spray-controlled combustion. The advantage with this is that a more robust combustion process is obtained, which can handle all load ranges, at the same time as ultra-low emission levels are obtained.
In an advantageous third embodiment of the method according to the invention, the mixing is carried out globally, since essentially the entire quantity of fuel corresponding to one combustion cycle is injected and mixed in the cylinder before ignition and combustion are realized. This embodiment, too, can handle all load ranges, at the same time as ultra-low emission levels are obtained. This embodiment has many characteristics in common with HCCI. The essential difference is that the injection method itself creates the necessary mixing. Unlike the prior art, this embodiment can also handle high engine load ranges, at the same time as ultra-low emission levels are obtained without a far too rapid, strength-limiting build-up of pressure in the cylinder. The reason why it has been possible to limit the pressure build-up lies in a combination of a sufficiently large quantity of EGR, sufficiently low temperature and sufficiently low pressure in the cylinder, which limits the rate of the chemical reactions leading to ignition and combustion.
Further advantageous embodiments of the method according to the invention can be gleaned from the appended patent claims.